The Radiative Signature of Upper Tropospheric Moistening

The extent to which the Earth's climate will warm as a result of increasing levels of atmospheric carbon dioxide is strongly dependent upon the response of water vapor in the upper troposphere. Climate models predict that the concentration of water vapor in the upper troposphere may double by the end of the century due to anthropogenic increases in greenhouse gases. Unfortunately, our ability to detect such changes is limited by the scarcity of observing systems with sufficient accuracy and longevity to identify global trends in upper tropospheric water vapor. Here we use GCM simulations and satellite radiance measurements from TOVS to demonstrate the presence of a coherent spectral signature of upper tropospheric moistening on interannual to decadal time scales. The observed moistening signature is consistent with model predictions and corresponds approximately to a constant relative humidity increase in upper tropospheric moisture over the last two decades. We further demonstrate that without such an increase in upper tropospheric water vapor, the model would be unable to reproduce the satellite-observed radiance record over this period.